Discussion of Background
Image forming apparatus, such as printers or photocopies, include a photosensitive member, typically in the form of a photosensitive drum. The performance of the photosensitive drum is of critical importance, since the image being produced (or reproduced) is formed and developed on the drum surface. The developed image is then transferred from the drum to, for example, a sheet of paper. Typically, the drum is formed of metal, such as aluminum, and the metal is anodized or coated to provide a thin dielectric layer. The drum is then coated with photogeneration and photoconduction layers over the dielectric layer.
In forming an image, the drum is rotated, and a given location on the outer surface of the drum is thereby rotated past a charging device, an exposure location, a developing location (at which toner is applied), a transfer location (at which the toner image is transferred from the drum to paper), and a cleaning location at which a cleaning blade removes excess toner from the drum so that the process can be repeated. During an image forming operation, as a result of the rotation of the photosensitive drum, and its interaction with the various other components of the image forming apparatus, noise and vibration can occur. This is particularly true since the photosensitive drum is a thin-walled metal drum, and thus has a characteristic harmonic sound spectrum which is easily driven by any mechanical resonance. For example, vibration (and associated noise) can occur from the rotation of the drum, and any imperfections of the drum, the gear flanges attached to the drum, and/or the drive which interacts with the gear flanges of the drum. Further, an alternating current (AC) electric field is applied to the charge roller, and the alternating current can also cause noise and/or vibration of the drum or between the drum and other components. In addition, as the drum rotates past the cleaning blade (which is in contact with the drum), noise is often generated, particularly if the drum surface is roughened by use. This interaction between the drum and cleaning blade is also known as chatter vibration or "stick-slip" vibration. (See, e.g., Chatter Vibration of a Cleaner Blade in Electrophotography, by Kawamoto, in the January/February 1996 issue of Journal of Imaging Science and Technology.) The noise and vibration associated with operation of a photoconductive drum not only presents an annoyance to workers using (or in the vicinity of) the image forming apparatus, but also, the noise/vibration can lead to image deterioration or damage to the apparatus. In particular, the vibration can result in poor performance or interaction between the photosensitive drum and one or more of the components with which the drum interacts, including the cleaning blade, the charge roller, the developer device, etc. Vibration may cause image blurring especially with the current trend to higher resolution devices (evolutions from 300 to 1200 dots per inch). For example, if the cleaning blade does not properly remove residual toner, undesirable resolution of character images can occur in subsequent images. Further, if the drum is not changed or developed properly, the resulting image can have white spaces where the image has not been properly formed, developed or transferred, or black spots where undesired toner has been transferred to the sheet of paper. Noise problems can also occur as a result of the generation of gases (ozone) which occurs during an image forming operation, however this noise is typically relatively small.
To eliminate noise and/or vibration, the physical characteristics of the drum can be modified, for example, by increasing the thickness of the drum. Thus, the drum can be designed so that its natural frequency differs from that of other components of the apparatus and/or that of the process cartridge (the unit within which the drum is disposes). As a result, the vibrations are eliminated or reduced, or the frequency of the noise which might occur can be shifted so that it is outside of the audible range. However, increasing the thickness of the drum can make the drum more expensive to manufacture, particularly if the tooling utilized to manufacture a drum must be replaced. In addition, thicker drums may have an undesirable increase in heat capacity and excessive weight causing higher inertia and drive torque. Moreover, when photosensitive drums are manufactured as replacement parts, they will often be inserted into process cartridge of another manufacturer. The process cartridge could be refurbished or a newly manufactured replacement process cartridge of a different manufacturer than that of the photosensitive drum, and the manufacturer/refurbisher of the process cartridge could change (or the design of a given manufacturer/refurbisher could change). Thus, it can be difficult to simply select a thickness of the drum which will be suitable for avoiding noise problems, since even if a thickness is selected for a certain process cartridge, that thicknes could be unsuitable for another process cartridge. As a result, noise problems can be particularly problematic with photosensitive drums manufactured as replacement parts.
A further difficulty which can arise with photosensitive drums is that the roundness or circularity of the drum can vary over time, which can also lead to image deterioration. The roundness or circularity of the drum can more rapidly deteriorate if the drum is vibrating and contacting other components disposed about the drum. This problem can also be reduced by providing a thicker drum, however as discussed above, increasing the thickness of the drum can increase the cost, from a materials standpoint and/or the requirement for new tooling.
An alternate solution which has been utilized in the past for solving noise and/or vibration problems has been to insert plugs within the photosensitive drum. U.S. Pat. No. 5,488,459 to Tsuda et al. discloses an example of such an approach. With this solution, a disk or cylindrical object is inserted into the drum, and the insert provides additional weighting to the drum to alter the mass/frequency characteristics of the drum. However, the use of plug-type inserts is undesirable for a number of reasons. First, the plug is often required to be positioned at a precise location within the drum, which can complicate the manufacturing process. Further, the plug must be secured in place, which can require the use of an adhesive, thus further complicating the manufacture/assembly process. An interference fit can also be provided between the drum and plug, however, an interference fit could result in deformation of the drum. Further, the plug and/or its associated adhesive can alter the performance characteristics of the drum. For example, as noted above, after a period of use, the circularity of the drum can deteriorate. The use of a plug can make the drum less uniform, since the plug will be located at a given position and will provide additional support, while portions of the drum spaced from the plug will not be supported. Further, the plug must be precisely manufactured. If it is too large, it could cause deformation of the drum, or be difficult to insert within the drum. If the plug is too small, it can be difficult to position the plug within the drum and secure the plug in place. Thus, the use of a plug or weight which is inserted inside of the drum has been less than optimal.
In view of the foregoing, a device and method are needed for reducing noise and/or vibration in image forming apparatus, particularly noise and/or vibration associated with operation of a photosensitive drum. Such a device and method are preferably suitable for use in both original equipment and for replacement parts.